They also go out less and party less.
The first time they hurt themselves, they experience unpleasant pain.
After the initial diagnostic tests, the researchers followed up at year five and at year 10 with a full day of clinical assessments of each girl.
Waddell points out that - as with anorexia, where many adolescents start off by dieting, and only a few develop eating disorders - it is hard to distinguish between the serious and casual act of cutting.
Why was the increase in self-harm so much larger for girls than boys? A teen who cuts himself or burns himself begins to focus on the injury as the reason for the pain.
You might be the first person they have been able to talk to about this.